The performance of mechanical components which have to support either static or cyclic loads can be critically affected by residual or applied stresses, because these stresses affect their load limits and fatigue life. The presence and magnitude of these stresses can therefore be a concern in terms of safety, life or reliability of the component. On the other hand, there are instances in which the presence of residual stresses is desirable and thus they are induced by known means. In both cases, information on the magnitude of such stresses is required.
Prior art devices can obtain quantitative evidence of residual stresses by using x-ray lattice spacing determinations, or via ultrasonic bulk or surface wave velocities. For applied stresses, strain gauges are also used. All these techniques are difficult to execute, with usually stringent prerequisites and requirements.
The prior art machines have problems. X-ray machines usually limit the dimensions of the components to be tested. Ultrasonic bulk wave techniques require parallelism of two opposite surfaces, and work on the assumption that the measured stress is constant between those surfaces. The use of surface wave ultrasonic velocities require smooth surfaces and usually relatively large distances are needed for the required accuracy. Again, the stress must be constant along the path used to measure the surface wave velocity, since the wave should observe constant stresses over its travel. Often the components to be tested are such that the requirements for measurement are not met, i.e. the flat surface is not large enough for placement of an ultrasonic transducer, or a sufficiently large smooth surface is not available for sufficient resolution in the residual stress determination via surface waves, or two surfaces are not sufficiently parallel for the bulk wave determination.